Java Day_5

JIT Compiler (Just-In-Time Compiler)

• Part of the JVM execution engine.
• Purpose: Improve performance of Java programs at runtime.

🔹 How JVM works without JIT:

• JVM uses an interpreter, which executes bytecode line by line.
• This makes execution slow.

🔹 How JIT works:

• JIT compiles frequently used methods/instructions (called hotspots) into native machine code.
• Instead of interpreting repeatedly, the JIT compiler stores compiled machine code.
• At execution time, JVM directly executes the native machine code → faster performance.

Variable in Java

• A variable is a name given for a memory location.
• It can hold some meaningful value within its range.
• A variable’s value can change during program execution.

Example:

int x = 10;
int y = 20;
x = x + y;

Memory Representation (RAM Example):

x → stores value at address (e.g., 1002).

• y → stores value at address (e.g., 1006).

Word Breakdown:

• Variable = “Vary + able” → changeable.

✅ Key Points:

• JIT Compiler = improves performance by compiling bytecode into machine code at runtime.
• Variable = named memory location whose value can change during execution.

🔹 Drawbacks of an Ordinary Variable

• A variable can store only one value at a time.

int x = 12, 90; // Invalid

• Stored in random memory location.
• Volatile → when program ends, variable’s value is deleted from memory.
  👉 To store permanently, we use File Handling.

🔹 What is Data Type?

• Definition: Specifies the type of data a variable can store.
• Describes:

1. Size — memory required in RAM.
2. Range — values it can hold. (Example: byte → -128 to 127).
3. operations — allowed operations (addition, string concatenation, etc.).

🔹 Types of Data in Java

Java has 2 main categories:

1. Primitive Data Type

Also called: primary, predefined, basic, fundamental, atomic, intrinsic.

• Total: 8 types
• Numeric/Integral
• byte (8 bits)
• short (16 bits)
• int (32 bits)
• long (64 bits)
• Decimal
• float (32 bits)
• double (64 bits)
• Non-Numeric
• char (16 bits)
• boolean (1 bit or depends on JVM)

2. Reference Data Type

Examples: class, array, interface, enum, record, etc.

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✅ Summary:

• Ordinary variables → store only one value, temporary.
• Data type → defines size, range, and operations of data.
• Java has 8 primitive data types and reference data types.

🔹 Statically Typed vs Dynamically Typed Languages

Statically Typed Language

• Data type must be declared before using a variable.
• Once declared, the variable holds the same type of value till program ends.
• Example: C, C++, Java (JSE), C#

👉 Example in Java:

int x = 10;  // must declare int
x = "Hello"; // ❌ not allowed

Dynamically Typed Language

• Data type not required when declaring variables.
• Variable can hold different types of values during execution.
• Exaple: Python, JavaScript, Visual Basic

👉 Example in Python:

x = 10
x = "Hello"  # ✅ allowed

🔹 Comments in Java

• Used to make code readable.
• Ignored by the compiler.

Types of Comments:

1. Single-line Comment

// This is a single-line comment

1. Multi-line Comment

• /* This is a multi-line comment */

1. Documentation Comment (for projects & APIs)

/** * Name of Project: Online Shopping * Modules: 70 * Project Date: 12th March 2024 * Last Modified: 9th Sep 2025 * Author: James Goling and his friends */

✅ Summary:

• Statically typed → type fixed (C, Java).
• Dynamically typed → type flexible (Python, JavaScript).
• Comments → make code readable (3 types: single, multi, documentation).